Gas turbine rotors are typically formed by stacking the rotor wheels and spacers axially one against the other. Bolt holes are provided through the wheels and spacers and receive bolts which are used to finally secure the wheels and spacers to one another to form the rotor. The wheels and spacers in final assembly also have rabbeted joints. That is, axially projecting flanges formed on the spacers underlie and fit tightly against axially oppositely extending flanges formed on the wheels. To form the rabbeted joints, the wheels are typically heated in an oven prior to assembly in the stack to expand the flanges of the wheels so that, after stacking and upon cool-down, the flanges of adjacent wheels and spacers fit tightly relative to one another.
During stack-up of the wheels and spacers, the bolt holes of the wheels and spacers are fitted over bolts projecting from a fixture. The bolts remain in the rotor assembly and maintain the wheels and spacers stacked relative to one another. Consequently, to enable the stack-up of the wheels and spacers on the bolts, substantial clearances between the bolt holes through the wheels and spacers and the bolts are necessary in the radial direction and corresponding clearances are therefore also provided in the circumferential direction. A need has developed, however, for a much tighter alignment of the stacked wheels and spacers which cannot be provided by the alignment of the bolt holes and bolts during the assembly stack-up consistent with the need to accommodate radial expansion and contraction of the heated wheels during the stack-up.
This need has arisen as a result of a new advanced steam-cooled gas turbine design of the assignee of the present invention wherein certain parts of the rotor are steam-cooled. In this advanced steam-cooled turbine design, a plurality of openings, in addition to the bolt holes, are provided through the wheels and spacers of the rotor to accommodate a plurality of circumferentially spaced tubes extending generally axially through the rotor for supplying steam to the steam-cooled parts, i.e., first and second stage rotor buckets, and returning the spent cooling steam to the rotor bore assembly. The supply and return tubes are thin-walled structures which extend through openings in bushings provided in circumferentially spaced apertures of the stacked wheels and spacers. Tight clearances between the tubes and bushing openings are highly desirable. The steam-carrying tubes desirably have as large a diameter as possible to maximize steam flow, as well as have very tight clearances with the openings to preclude high stresses on the tubes. Thus, there is a need to tightly tolerance the openings through the wheels and spacers which carry the steam-cooling tubes while concurrently enabling radial contraction of the wheels to form tightly rabbeted joints.